1. Field of the Invention
The present invention relates to the formation of high density integrated circuits and, more particularly, to the formation of high density dynamic random access memories.
2. Description of the Related Art
There is a continuing trend toward increasing the storage density of integrated circuit memories to provide increased levels of data storage on a single chip. Higher density memories provide storage that is generally more compact and is often cheaper on a per bit basis than an equivalent amount of storage provided on plural chips. It has generally been possible to provide these higher levels of storage at equivalent or improved levels of performance as compared to the earlier, less dense memory chips. Historically, the density of integrated circuit devices has been increased in part by decreasing the size of structures such as wiring lines and transistor gates as well as by decreasing the separation between the structures that make up the integrated circuit device. Reducing the size of circuit structures is generally referred to as decreasing the "design rules" used for the manufacture of the integrated circuit device.
In dynamic random access memories (DRAMs), information is typically stored by selectively charging or discharging each capacitor of an array of capacitors formed on the surface of a semiconductor substrate. Most often, a single bit of binary information is stored at each capacitor by associating a discharged capacitor state with a logical zero and a charged capacitor state with a logical one, or vice versa. The surface area of the electrodes of the memory capacitors determines the amount of charge that can be stored on each of the capacitors for a given operating voltage, for the electrode separation that can reliably be manufactured, and for the dielectric constant of the capacitor dielectric used between the electrodes of the charge storage capacitor. Read and write operations are performed in the memory by selectively coupling the charge storage capacitor to a bit line to transfer charge either to or from the charge storage capacitor. The selective coupling of the charge storage capacitor to the bit line is accomplished using a transfer field effect transistor (FET). A contact between the bit line and the transfer FET is made to one of the source/drain electrodes of the transfer FET and the charge storage capacitor is formed in contact with the other of the source/drain electrodes of the transfer FET. Word line signals are supplied to the gate of the FET to selectively connect the lower electrode of the charge storage capacitor through the transfer FET to the bit line contact, facilitating the transfer of charge between the charge storage capacitor and the bit line.
Applying reduced design rules to a DRAM reduces the substrate surface area that can be devoted to the charge storage capacitors of the DRAM. Thus, applying reduced design rules to conventional planar capacitor designs reduces the amount of charge (i.e., capacitance) that can be stored on the charge storage capacitor. Reducing the amount of charge on the capacitor leads to a variety of problems, including the potential loss of data due to greater susceptibility to decay mechanisms and to charge leakage. This greater susceptibility to charge loss may cause the DRAM to require more frequent refresh cycles, which is undesirable since the memory may be unavailable for data storage and readout transactions during refresh activities. In addition, reduced levels of charge storage might require more sophisticated data readout schemes or more sensitive charge sensing amplifiers. Thus, modem DRAMs require increased levels of capacitance in reduced substrate area DRAM cells. To this end, a variety of very complex capacitor structures having three dimensional charge storage surfaces have been proposed. In general, these complex capacitor structures are difficult to manufacture.
One strategy that has been adopted to provide increased levels of capacitance to DRAM capacitors has been to deposit hemispherical grained silicon over the surfaces of capacitor electrodes to enhance their surface area. Hemispherical grained silicon ("HSG-Si"), sometimes referred to as textured polysilicon, is a phase of silicon deposited under precise deposition conditions and that has a significant level of surface roughness. For example, HSG-Si can be deposited in an LPCVD process at substrate temperatures of between about 555.degree. C. to 590.degree. C., depending on the particular pressure range and system used. For a particular combination of processing conditions, however, it may be necessary to maintain temperature control to within .+-.5.degree. C. to achieve desired deposition characteristics. Depositing HSG-Si can introduce a surface roughness on the order of about 50-100 nanometers, significantly more than the typical roughness of a polysilicon surface. Providing a layer of HGS-Si on an otherwise planar layer of polysilicon can increase the surface area of the structure by a factor of about 1.8 and so can increase the capacitance of a DRAM charge storage capacitor. On the other hand, it is difficult to achieve any higher increase in capacitance through the deposition of HSG-Si on its own. It is difficult to justify the increased process complexity, process variability and reduced yields associated with the use of HSG-Si solely on the basis of a factor of 1.8 improvement in capacitance.